i think i was about 7 when i had a frightening dream that my reflection came out of the mirror and caused all kinds of trouble for me, evil twin style. after that i was terrifyed that it would really happen, and checked on my reflection quite often to make sure it wasn't planning on anything.
A childhood belief about mirrors

others believed things like this
- I used to believe that my reflection in the mirror was my evil twin that always had bad hair. XDAnon
- i always thought I was the reflection and your reflection was a personAnon
- I used to believe that when you cracked a mirror your reflection would come and take over your body!— XxXAbbeyXxX
- If you stare into a mirror in almost complete darkness, the devil will appear instead of your reflection.Anon
- I used to think I could sneak up on my reflection and catch myself not looking back at me.— Mary Poppins
- If you held a buttercup under your chin and got a yellow reflection then you liked butter!— Tilly